1. Field of the Invention
This invention relates to Raman optical amplifiers.
2. Discussion of the Related Art
To compensate attenuation, optical communication systems often provide for amplification of optical signals at regular intervals along optical transmission fibers. The amplification may be produced by amplifiers based on rare-earth elements such as erbium and ytterbium or by amplifiers based on the Raman effect. Rare-earth amplifiers have limited bandwidth due to their reliance on selected atomic level transitions. Amplification occurs at discrete wavelengths that correspond to the selected atomic transitions. Broadband erbium doped fiber amplifiers are somewhat improved rare earth amplifiers so that these rare-earth amplifiers can power some wavelength division multiplexed (WDM) optical networks. On the other hand, Raman amplifiers are naturally tunable and capable of providing amplification at wavelengths in a broad optical band. In such an amplifier, an amplification wavelength is simply selected by tuning a pump laser to produce a wavelength capable of producing stimulated Raman emission at the selected wavelength. Raman amplifiers can cover a much wider spectral range than rare-earth based amplifiers. Furthermore, Raman amplifiers have effectively lower noise levels than rare-earth amplifiers. These advantages make Raman amplifiers desirable for long haul WDM systems where the transmission bandwidth may be broad.
Nevertheless, conventional Raman fiber amplifiers provide relatively low gain. In such amplifiers, an optical signal often has to propagate through a long and heavily pumped amplifier fiber to receive adequate amplification. For example, to produce a 20-dB amplification, some conventional Raman fiber amplifiers use 10 to 100 kilometers (km) of amplifier fiber and 300 to 1,000 milli-Watts (mW) of pump light. High pump light powers require expensive pump lasers and incur higher operating costs for pump lasers. Raman amplifiers based on shorter amplifier fibers and lower pumping powers are desirable.
In one aspect, the invention features an optical amplifier including a chalcogenide glass optical waveguide with optical input and output ports, a pump optical waveguide, and a wavelength-tunable pump laser. The pump optical waveguide couples the wavelength-tunable pump laser to the chalcogenide glass optical waveguide.
In a second aspect, the invention features a method of amplifying light. The method includes tuning a wavelength-tunable pump laser to produce pump light with a wavelength capable of causing Raman amplification in a chalcogenide glass optical waveguide in response to light of a selected wavelength being received in the chalcogenide glass optical waveguide. The method also includes delivering the pump light to the chalcogenide glass optical waveguide, and receiving input light with the selected wavelength in the chalcogenide glass optical waveguide.
In a third aspect, the invention features an optical communication system. The system includes a plurality of silica glass optical fibers and at least one Raman amplifier coupled between two of the silica glass optical fibers. The Raman amplifier of the present invention includes a chalcogenide glass optical waveguide connecting the two of the silica optical fibers, a pump optical waveguide, and a wavelength-tunable pump laser. The pump optical waveguide couples the pump laser to the chalcogenide glass optical waveguide.